The invention relates to methods and apparatus for indicating the position of the closure members of a subsea blowout preventer (“BOP”).
A BOP is a safety device that closes, isolates, and seals a wellbore to prevent the uncontrolled release of crude oil and/or natural gas from the well. One common type of BOP, a ram-type BOP, uses two opposed closure members, or rams, disposed within a specially designed housing or body having a bore aligned with that of the wellbore. Opposed cavities intersect the bore and support the rams for movement into and out of the bore. A bonnet connected to the body on the outer end of each cavity supports an operator system that provides the force required to move the rams into and out of the bore. The force is usually provided by pressurized hydraulic fluid.
The rams are equipped with sealing members that engage with one another to prohibit flow through the bore when the rams are closed. The rams may be pipe rams, which are configured to close and seal an annulus around a pipe disposed within the bore, or the rams can be blind or shearing rams, which are configured to close and seal the entire bore, and can shear a pipe in the wellbore. A particular drilling application may require a variety of these pipe rams and blind rams assembled as a stack of BOPs.
One issue with ram-type BOPs when used subsea is knowing, with certainty, whether the rams have fully extended once the BOP is activated. “Inside the box” approaches to indicate the position of the rams install electronics directly into the pressurized operating chambers of the BOP and other well control equipment. These installations tend to be complex and the installed electronics can sometimes compromise pressure integrity of the operating chambers.